Figure 3-12. Visual Limitations

TM 9-258

b. Resolving Power of the Eye. The retina is a

lines can be distinguished by adjacent cone endings

closer than displacement of points. Some persons can

mass of nerve endings. Their individual diameters

read accurately angular measurement (between two

(between 0.0015 millimeter and 0.0054 millimeter)

displaced lines) as small as 10 seconds of arc.

determines the resolving power of the particular eye.

d. Measure of Visual Acuity. Visual acuity is

The size of these nerve endings vary in different

keeness of sight. It is the ability to see near and distant

individuals. These nerve endings may be compared to.

objects clearly in order to compare minute details and to

the sensitive particles in a photographic film. Although a

discriminate between them. Visual acuity commonly is

perfect image may be formed in the retina, it will not be

measured by viewing a standard letter of one-minute

perceived as such, for the distance between two

details, such as the following example: A person able to

adjacent image points must be greater than the distance

view the letter E composed of lines one minute of angle

between rods or cones (nerve ends) for perception.

c. Acute Vision. Acute vision is limited to the

of width at the standard distance of 20 feet is said to

possess 20/20 vision. In case a letter is used at 20 feet

area of the fovea centralis, a spot (about 0.25 millimeter

which should normally be identified at 40 feet, vision is

in diameter) on the macula which lies on the retina at the

said to be 20/40. Some eyes, on the other hand, have

visual axis. Here are cone nerve endings alone instead

better than average resolving power and can perceive

of the longer rods with a diameter of approximately 0.002

letters having less than one-minute details. An example

millimeter. This area of acute vision covers a true field of

of this is 20/10 vision, which it the ability to discern at 20

less than 1 of arc (ordinarily) or a circle approximately

feet what the normal eye can appreciate at 10 feet. A

3.5 millimeters in diameter at 25 centimeters distance

whole series of letters have been designed (subtending

from the eye. Least angular separation between any two

one-minute details at their respective distances) for use

discernible points in this field of acute vision is normally

at the standard distance, so that the testing can be done

one minute of arc (B, fig 3-12). Coincident readings, as

at a fixed distance rather than at various distances.

on a vernier or micrometer, can be read closer as

angular

displacement

two

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